Fig. 3. (Color online) Normalized output from the launch arm of a 20-cm-long coupler. The undoped coupler (solid) crosses the 50% region rapidly and therefore has a narrow bandwidth. The microstructured coupler has a plateau around 50% centered at 1:4 THz and therefore has a broad bandwidth at 1:4 THz. In this case, dc=Λc ¼ 0:105.
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